Field of the Disclosure
The present disclosure relates to an overload protection for a loadcell probe so that a specimen or device under test is protected when excessive forces are applied by the probe during materials testing.
Description of the Prior Art
Materials testing machines used to measure force and displacement of compliant surfaces under compression use a loadcell force sensor with a probe to contact the specimen under test. The probe is driven into the specimen with a precision linear drive system and the displacement is measured with a linear or rotary position encoder. The materials testing machine has software designed to command and control the position of the probe with force feedback provided by the loadcell via the probe contacting the compliant specimen. There are some applications, typically in automated environments such as assembly lines in factories, whereby the load testing machine is connected to external software that initiates motion commands to start and stop tests as well as to collect and process the materials testing machine's test results.
There can be occasions where the control of the drive axis can become abnormal and unpredictable. One example of abnormal behavior of the system is where the drive system that positions the probe into the specimen unintentionally drives “open loop” at high rate of speed introducing a very high load into the specimen. This scenario can damage the specimen under test or nearby equipment and/or represents a safety risk to people around the test equipment. There are typically several software and electronic speed and load limits in place on the materials testing machine that are expected to stop this inadvertent out of control motion. In some cases, however, the out of control condition can either occur too quickly or during a state of non-communication or “hanging up” condition by the control software whereby the software or electrical limits cannot react in time and the crash event cannot be prevented through the normal means.
Some possible causes of the drive system's motion to become unpredictable and lose control could be:                Failure in a drive system hardware component (motor, encoder, power amplifier, cables)        Software bugs        Incompatible interaction between the testing machine's software, hardware and firmware        Interaction with the factory's software that commands and controls the load test machine        Environmental conditions—operators, power spikes or surges.        
A common method to limit the torque to a motor is to use an electrical or mechanical friction slip clutch in between the motor and drive shaft. This approach was not satisfactory for this design because it is not at the point of application of the load/specimen and because the system draws high currents during its normal operation—particularly when the drive system needs to accelerate and decelerate. Additionally, a friction clutch could slip under these conditions, which would be undesirable.